1. Field of the Invention
The present invention relates to a laser scanning position detecting device for detecting the scanning position of a scanning laser beam supplied from a laser unit, which is used in a laser printer to expose an electrostatic latent image forming member such as a photosensitive material drum of a photosensitive material belt.
2. Description of the Related Art
A laser printer of tandem type has been known. In this type of laser printer, exposure optical systems (laser units) corresponding to four printing colors, i.e., yellow (Y), magenta (M), cyan (C), and black (Bk), are disposed in parallel to the rotation direction of the photosensitive material belt having an electrostatic latent image formed thereon. By operating these exposure optical systems in synchronism with the rotating movement of the photosensitive material belt, an image (an electrostatic latent image) is formed on the photosensitive material.
FIG. 1 is a schematic configuration diagram showing an image forming section of a conventional laser printer of tandem type having a photosensitive material belt and four laser units. In the printer shown in FIG. 1, however, illustration of a developer is omitted.
The laser printer of tandem type shown in FIG. 1 includes a photosensitive material belt 1501 formed of a belt-shaped photosensitive material on which an electrostatic latent image is formed, and four laser units corresponding to four printing colors (a laser unit 1502 for the Y color, a laser unit 1503 for the M color, a laser unit 1504 for the C color, and a laser unit 1505 for the Bk color). Four laser units are disposed in a line along a rotation direction of the photosensitive material belt 1501.
The photosensitive material belt 1501 is wound around a drive roller 1507 and a steering roller 1508. The photosensitive material belt 1501 is adapted to be rotated when rotating drive force of a motor 1506 is transmitted to the drive roller 1507 via a transmission system such as gears which are not illustrated. The steering roller 1500 is provided to prevent the photosensitive material belt 1501 from meandering. As a result, the equilibrium state of the photosensitive material belt 1501 is maintained.
In the tandem-type printer having such a configuration, mounting precision of the laser units for forming respective colors is important. In the tandem-type printer, therefore, an L-shaped installation positioning plate 1509 is provided as a means for mounting the laser units for respective colors with high precision. Hereafter, an alignment process of laser units using an installation positioning plate 1509 will be described.
Typically, in each laser unit, there is an SOS (Start of Scan) sensor which serves as an image writing start reference and which is not illustrated. By taking a signal output of this SOS sensor as a reference, image forming timing is derived. The SOS sensors in respective laser units are adjusted so as to have the same image forming timing at predetermined installation positions of the respective laser units. When the laser units for respective colors are fixed on the same installation positioning plate 1509, therefore, positions of respective laser units are adjusted by using an adjustment mechanism while measuring the intervals between the laser units in the installation positioning plate 1509 side and the intervals between the laser units in a side opposite to the installation positioning plate 1509. As a result, the mounting precision of respective laser units is ensured, and alignment of respective laser units is conducted.
However, the laser units can be aligned by using the above described means only in the case where the laser units are disposed so as to be parallel to each other as in the printer shown in FIG. 1.
As shown in FIG. 2, however, in the case where it is necessary to swell an exposed surface of a photosensitive material belt 1601 in an arch form from restrictions concerning the reduction in apparatus size and the configuration of a developer, the laser units 1602 cannot be mounted so as to become parallel to each other. In this case, it is difficult to accurately measure the mounting error between the laser units as described above. Even when the mounting error can be measured, differing mounting angles of the laser units cause different positions of laser beam scan lines of the laser units 1602 with respect to the vertical direction, due to the relation of bearing structures supporting polygon mirrors 1602a respectively included in the laser units 1602. With respect to the installation positioning plate of the laser units, therefore, the scan lines are not in the same position (height).
Furthermore, even in the case where the laser units are arranged so as to become parallel to each other, the laser beam emitting faces of respective laser units need to be inclined uniformly by an angle of approximately 5 degrees with respect to the surface of the photosensitive material belt in order to prevent laser beams reflected by the surface of the photosensitive material from returning to the laser units. Also because of necessity of providing this inclination, it becomes difficult to keep the positioning precision.
Due to these facts, it is necessary to be able to detect where the scan line actually scanned on the photosensitive material passes through. Heretofore, however, there has not been a sensor capable of detecting the scanning position of laser beam which is scanning at high speed. Therefore, there has been used a technique of forming a test pattern for detecting a position deviation value of the scan line on the photosensitive material, transferring the test pattern on paper, reading the position deviation value of the scan line from an image obtained on the paper, and adjusting the positions of the laser units.
In the case where the above described technique is used as the method for detecting the mounting states of the laser units, all image forming processes must be incorporated in the near perfect state. It is necessary to derive the position deviation value occurring at this time, as a numerical value, from the image transferred onto paper. Furthermore, since the contrast of the color Y is low, it is difficult to automate reading the position deviation value of the scan line for each color by taking the color Y as the reference. It is necessary for a person to judge the line center of the drawn test pattern by watching and read the distance between the centers as a numerical value of the position deviation value. A great deal of labor is required for the position adjustment of each laser unit.
In Japanese Patent Application Laid-open No. 5-119572, a color image forming apparatus having a photo sensor separated from a laser unit is disclosed. The photo sensor is installed at one edge of a photosensitive belt and is used to sense a laser scanning beam at the one edge to produce a image write timing signal.
In Japanese Patent Application Laid-open No. 4-163515, a laser beam printer is provided with a light-receiving portion having a light-gathering lens for gathering an incident laser beam to produce a beam spot on a photo sensor for a predetermined time period. The output of the photo sensor is used to obtain a image write timing.
However, these devices disclosed in Japanese Patent Application Laid-open Nos. 5-119572 and 4-163515 are directed to detection of the write timing when forming an image. Therefore, they cannot provide detection of the scanning position of a laser beam for adjustment of installation position of a laser unit.
In view of the foregoing description, an object of the present invention is to provide a laser scanning position detecting device which allows detection of the scanning position of a laser beam scanning at high speed.
Another object of the present invention is to provide a laser scanning position detecting apparatus which allows a laser unit to accurately place at a predetermined position.
Still another object of the present invention is to provide a laser scanning position detecting device which allows detection of the incidence timing and the scanning position of a laser beam scanning at high speed.
According to the present invention, a device for detecting a scanning position of a laser beam is used for a laser unit which scans a photosensitive material member with the laser beam to form an electrostatic latent image thereon. The device includes a position detector for detecting the scanning position of the laser beam from a beam spot of the laser beam formed thereon when the laser beam passes through the device and an optical system for directing the laser beam to a predetermined point of the position detector to form the beam spot for a predetermined time period during which the laser beam is scanning the device.
The device may further include a timing detector for detecting timing of incidence of the laser beam to the device. The optical system preferably comprises a lens having a first option and a second portion, wherein the first portion instantaneously directs the laser beam to the timing detector and the second portion directs the laser beam to the predetermined point of the position detector.
The optical system may include a slit through which the laser beam travels to the timing detector. The optical system may include a first lens for instantaneously passing the laser beam to the timing detector and a second lens for directing the laser beam to the predetermined point of the position detector. The optical system may include a slit through which the laser beam instantaneously travels to the timing detector and a lens for directing the laser beam to the predetermined point of the position detector.
The optical system may direct the laser beam to the predetermined point of the position detector such that the laser beam directed is incident to a light-receiving surface of the position detector in a direction approximately perpendicular to the light-receiving surface.
As described above, according to the present invention, the laser beam is directed onto the predetermined point of the position detector while scanning the optical system. Such a state that the laser beam is incident to the position detector as if to be quiescent thereon is made for the predetermined time period. Therefore, a stable output of the device can be obtained, ensuring the precise detection of a scanning position of the laser beam scanning at high speed.
Further, since the timing of incident laser beam can be detected by the timing detector, it is possible to sample and hold an output signal of the position detector at the time when the output signal become stable. Furthermore, since the optical system instantaneously directs the laser beam to the timing detector, the incident timing of the laser beam can be detected sharply.
According to another aspect of the present invention, a scanning position detecting apparatus comprises a first sensor provided at a scanning start location of the laser beam emitted from the laser unit and a second sensor provided at a scanning end location of the laser beam emitted from the laser unit. The first sensor includes a first position detector for detecting a first scanning position of the laser beam from a beam spot of the laser beam formed thereon when the laser beam passes through the first sensor, and a first optical system for directing the laser beam to a predetermined point of the first position detector to form the beam spot for a predetermined time period during which the laser beam is scanning the first sensor. The second sensor includes a second position detector for detecting a second scanning position of the laser beam from a beam spot of the laser beam formed thereon when the laser beam passes through the second sensor, and a second optical system for directing the laser beam to a predetermined point of the second position detector to form the beam spot for a predetermined time period during which the laser beam is scanning the second sensor.
Since the respective outputs of the first and second sensors located at both ends of the scan range are used to detect a change of laser beam scanning position, the scanning position can be more precisely detected.